My sister is so hateful towards my mom

I just don't understand it! My sister has become so hateful towards my mom it's unbelievable. My mom is sick they gave her less than a year to live due to kidney failure plus she has other health problems so a transplant or dialysis won't work ( not looking or wanting sympathy). She watches my sister's kids for $100 every 2 weeks 5 sometimes 7 days a week. She watches them from 4 till 2 am maybe longer. Well my mom was using my sister's car ( they have 3 cars and only drive one) my mom was going to take over payments on the car shes been using starting in July. Well yesterday my mom me and my brother took the kids to the beach. My mom didn't want to drive she wanted to relax ( she wasn't feeling good) so my brother drove ( he's the oldest out of all of us kids) well they drop me off and go to drop my sister's kids off to her. They get there she takes her kids didn't say anything to my mom about this next part. My mom and brother leaves and my sister text my brother ( not my mom who has a phone) and tells him to tell my mom she needs her car for training at work my mom calls her and asks why she goes off saying " I just told him why" my mom asks for how long and she says idk a week , two week , a month maybe 3 months and that she wants it back today. My sister knows this is my mom's only transportation. My mom spends all the money they give her for watching them on the kids because my sister don't send them with food or anything and expects them to just sit in the house all summer instead of going places like the park or beach. My sister didn't even get my mom a card for mother's day but they went and bought her boyfriend's dad a tool set and new tires for his motorcycle for father's day. They bring home over $2000 every 2 weeks so it's not like they are broke or hurting for money. It just makes me so mad how they are doing her. If I had they money myself I would get her a car but I can't even afford one for myself seeing I'm 40 weeks pregnant and can't work until baby is born. My sister has started to hate my mom and I don't understand why. She has done so so much for her and her kids